(i) Field of the Invention
The present invention relates to a method for removing carbon monoxide (CO) from a reformed gas for the purpose of feeding a gas having a low CO concentration and a high hydrogen concentration to a system in which a gas containing hydrogen as a main component obtained by reforming a hydrocarbon such as methane or propane, or a hydrocarbon having an oxygen atom such as methanol or ethanol is used as a fuel gas for a fuel cell. In particular, the method of the present invention can be suitably applied to a solid polymeric type fuel cell in which the low CO concentration is required.
(ii) Description of the Related Art
In a phosphate type fuel cell or a solid polymeric type fuel cell, a reformed gas containing hydrogen as a main component is obtained on the side of a hydrogen pole by a fuel reformer. As a fuel for the fuel reformer, there can be used a fossil fuel such as methane, propane or naphtha, and such a fuel is reformed with water vapor or the like to obtain a reformed gas, or alternatively methanol or ethanol may be subjected to a water vapor reforming treatment to obtain the reformed gas. However, in the reformed gas, about 1% of CO is contained, and this CO poisons Pt which is used as an electrode catalyst, whereby the performance of the fuel cell deteriorates inconveniently. Particularly in the solid polymeric type fuel cell, its operation temperature is as low as 100.degree. C. or less, so that the influence of the CO poison is serious.
In order to eliminate the influence of the CO poison, it is necessary that the CO concentration should be 10 ppm or less, preferably several ppm or less.
For some means for solving these problems, the following techniques have been heretofore disclosed.
Japanese Patent Application Laid-open No. 201702/1993 discloses a technique that a small amount of air is added to the reformed gas to oxidize CO into CO.sub.2.
Furthermore, in Japanese Patent Application Laid-open No. 325402/1992, there has been described a method in which hydrogen alone is allowed to permeate through a hydrogen high-selective film such as a Pd alloy film, whereby the feed of CO to a fuel cell can be inhibited.
In addition, Japanese Patent Application Laid-open No. 251104/1993 discloses a technique that CO is hydrogenated to convert the same into methane.
However, in the technique disclosed in Japanese Patent Application Laid-open No. 201702/1993 in which air is added to selectively oxidize CO, the addition of air lowers the partial pressure of hydrogen to deteriorate the efficiency of the fuel cell. In addition, there are a problem that a pump for feeding air is required and another problem that the danger of explosion is always present.
In the method disclosed in Japanese Patent Application Laid-open No. 325402/1992 in which hydrogen alone is allowed to permeate through a hydrogen high-selective film such as the Pd alloy film, whereby the feed of CO to the fuel cell can be inhibited, a gas having a high hydrogen concentration can be obtained, which indicates that this method is excellent. However, if pinholes having the size of a gaseous molecule level exist in the Pd alloy hydrogen separating film, or if pinholes occur by a certain external load, CO in the reformed gas permeates through the film and reaches the fuel cell inconveniently on occasion.
In the technique in which CO is hydrogenated to convert the same into methane, the reaction of CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O is carried out to form methane, but CO still remains in an amount of several ppm. Thus, it is difficult to decrease the CO concentration to several ppm or less only by the catalytic reaction. It has been disclosed in Japanese Patent Application Laid-open No. 251104/1993 that after CO is oxidized to lower the CO concentration to some extent, the conversion of CO into methane can be done to decrease the CO concentration to 0 ppm. However, the disclosed process is complex and a large apparatus is required inconveniently. According to the process which has been disclosed herein, about 20% of CO.sub.2 contained in the reformed gas is also fed to the fuel cell, so that the partial pressure of hydrogen is low, with the result that a power generation efficiency is low.